Coal desulphurization

ABSTRACT

A process for reducing the pyritic sulphur content of coal includes reacting the coal with a ferric ion solution to oxidise the pyritic sulphur to elemental sulphur, and separating the coal, which includes elemental sulphur from the ferric ion solution. The coal and the elemental sulphur are then reacted in an acidic solution with nitric acid to oxidize the elemental sulphur, forming sulphuric acid. The coal is separated from the acidic sulphuric acid containing solution to provide coal with a reduced pyritic sulphur content.

This invention relates to desulphurization of coal. In particular, theinvention relates to a process for reducing the pyritic sulphur contentof coal.

Many coal sources provide coal with a high sulphur content. Forenvironmental reasons, it is undesirable to burn the coal and thereby torelease SO2 into the atmosphere.

It would be advantageous to provide an improved process to desulphurizecoal particularly if such a process does not negatively affect thecoking properties of the coal.

According to a first aspect of the invention, there is provided aprocess for reducing the pyritic sulphur content of coal, the processincluding

-   -   reacting the coal with a ferric ion solution to oxidise the        pyritic sulphur to elemental sulphur;    -   separating the coal, which includes elemental sulphur from the        ferric ion solution;    -   reacting the coal and the elemental sulphur in an acidic        solution with nitric acid to oxidize the elemental sulphur,        forming sulphuric acid; and    -   separating the coal from the acidic sulphuric acid containing        solution to provide coal with a reduced pyritic sulphur content.

The coal may be reacted with the ferric ion solution at a temperature ofless than 100° C. The nitric acid used to oxidise the elemental sulphurmay be at a concentration of less than 10% by mass.

According to a second aspect of the invention, there is provided aprocess for reducing the pyritic sulphur content of coal, the processincluding

-   -   reacting the coal with a ferric ion solution at a temperature of        less than 100° C. to oxidise the pyritic sulphur to elemental        sulphur;    -   reacting the coal and the elemental sulphur in an acidic        solution with nitric acid at a concentration of less than 10% by        mass to oxidise the elemental sulphur, forming sulphuric acid;        and    -   separating the coal from the acidic sulphuric acid containing        solution to provide coal with a reduced pyritic sulphur content.

Preferably, the coal is reacted with the ferric ion solution at atemperature of less than 90° C., more preferably less than 85° C., e.g.between 60° C. and 80° C.

The ferric ion solution may be a ferric sulphate solution, althoughferric chloride and ferric nitrate may also be used. Thus, the ferricion solution may be selected from the group consisting of a ferricsulphate solution, a ferric chloride solution, a ferric nitrate solutionand mixtures thereof.

Typically, reacting the coal with a ferric ion solution thus includescontacting the coal with the ferric ion solution by mixing the coal andthe ferric ion solution. As will be appreciated, an effective amount ofthe ferric ion solution must be used. The effective amount will depend,amongst other factors, on the amount of coal treated, the pyriticsulphur content of the coal being treated, and the ferric ionconcentration of the ferric ion solution.

The process may include adjusting the pH of the ferric ion solution tobe acidic. Preferably, the pH of the ferric ion solution is maintainedat less than 1. The pH of the ferric ion solution may be adjusted withsulphuric acid.

The nitric acid used to oxidise the elemental sulphur may be at aconcentration of less than 2.75% by mass, e.g. about 2.2% by mass.Typically, the nitric acid is at a concentration of at least 0.5% bymass.

The elemental sulphur may be reacted with nitric acid at a temperatureof less than 71° C., e.g. about 70° C. Preferably however, thetemperature is at least 40° C., more preferably at least 50° C.

The process in accordance with the second aspect of the invention mayinclude, after reacting the coal with a ferric ion solution, separatingthe coal, which includes elemental sulphur from the ferric ion solutionbefore reacting the coal and the elemental sulphur with nitric acid.Typically, the coal, which includes elemental sulphur, is filtered fromthe ferric ion solution and washed, e.g. with water, before reacting thecoal and the elemental sulphur with nitric acid. The process may thus bea two-stage process. In the first stage, pyritic sulphur is oxidized toelemental sulphur using a ferric ion solution and, in the second stage,the elemental sulphur is oxidized to sulphate sulphur, forming sulphuricacid, by reaction with nitric acid. Indications are that the two-stageprocess is more advantageous than a single-stage process.

Separating the coal from the acidic sulphuric acid containing solutiontypically includes filtering the coal from the acidic solution.Preferably, separating the coal from the acidic sulphuric acidcontaining solution includes washing the separated coal, e.g. withwater.

The invention will now be more fully described with reference to thefollowing examples and the accompanying diagrammatic drawings, in which

FIG. 1 shows a graph of percentage sulphur removed as a function ofnitric acid concentration for a single stage process and a double stageprocess in accordance with the invention; and

FIG. 2 shows a graph of percentage sulphur removed as a function oftemperature, for different nitric acid concentrations, for a singlestage process and a double stage process in accordance with theinvention.

EXAMPLE 1

This Example illustrates a single stage process in accordance with theinvention. A predetermined amount of ferric sulphate, typically between20 g/L and 300 g/L, was dissolved in about 1.8 liters of water and thesolution was heated to a predetermined temperature, e.g. 40° C. Thesolution was acidified with sulphuric acid to obtain a pH<1. 200 g ofsub-bituminous coal obtained from the secondary spiral product of thebeneficiation plant of Exxaro Resources, Grootegeluk, South Africa wasrapidly fed through a screw feeder, and sulphuric acid was again used tomaintain the pH below 1. The solution volume was raised with hot waterto reach the required temperature. The stating temperature of thesolution was selected such that the added sulphuric acid raised thetemperature to a desired temperature, e.g. about 70° C. Slurry sampleswere taken at predetermined time intervals using a glass pipette. Aftera predetermined reaction time, e.g. 30 minutes, nitric acid was added tothe solution to reach a required concentration, e.g. 2.2% nitric acid bymass. The acidic solution was kept at a desired temperature, e.g. 70° C.After the required reaction time, e.g. 15 minutes, the coal was filteredfrom the acidic solution and washed free of sulphur with water.

The reaction temperatures were controlled by conducting the reactions ina double walled leach reactor and controlling the temperature of waterflowing through a jacket of the double walled leach reactor. Aperistaltic pump was used to transfer the coal slurry from the doublewalled leach reactor to a filtration assembly. Probes were used tomeasure pH and reaction temperature. A vibrating feeder was used to feedthe coal into the double walled leach reactor.

EXAMPLE 2

This Example illustrates a two stage process in accordance with theinvention. A predetermined amount of ferric sulphate, e.g. between 20g/L and 300 g/L, was dissolved in about 1.8 liters of water and thesolution was heated, if necessary, to a temperature of about 30° C. Thesolution was acidified with sulphuric acid to obtain a pH<1. About 200 gof coal was rapidly fed through a screw feeder into a double walledleach reactor, and sulphuric acid was again used to maintain the pH<1.The solution volume was raised with hot water to reach the requiredtemperature. The starting temperature of the solution was selected sothat a desired final temperature is reached as a result of the additionof the sulphuric acid, e.g. 60° C.

After a predetermined reaction time, e.g. 30 minutes, the coal slurrywas removed from the leach reactor and filtered. The coal solids werewashed with water and then recycled to the double walled leach reactor.Nitric acid was injected into the double walled leach reactor to obtaina desired concentration, e.g. about 2.2% nitric acid by mass. Thetemperature was kept at a desired level, e.g. about 70° C. After therequired reaction time, e.g. about 15 minutes, the contents of thedouble walled leach reactor were again filtered, and the coal solidswashed free of sulphur with water.

In both Example 1 and Example 2, the desulphurized coal was dried andstudied with a scanning electron microscope. Samples of the acidsolutions from the double walled leach reactor were titrated with bariumchloride to determine sulphate sulphur content, using sulfonanzo III asindicator. Iron content of the solution samples was determined bycomplexometry with EDTA using 5-sulfosalicyclic acid as indicator. Totalsulphur was determined using Leco apparatus.

The effectiveness of the process in accordance with the invention forreducing the pyritic sulphur content of coal, using differentconcentrations of nitric acid and a single stage process or a doublestage process along the lines of Example 1 or Example 2, is illustratedin FIG. 1 of the drawings. As can be noted, the double stage process,with the ferric ion solution at 60° C. and the nitric acid solution at80° C., is considerably more effective at removing sulphur than a singlestage process operating at 80° C. The advantage of using a double stageprocess is marked at low concentrations of less than about 2.2% nitricacid and greater than 0.5% nitric acid. It is believed that the adhesionof ferric sulphate to the coal and the loss of nitric acid as ferroussulphate are minimized at high acid concentrations and hightemperatures.

FIG. 2 illustrates the effectiveness of the process of the invention inreducing the pyritic sulphur content of coal, using a double stageprocess at two different nitric acid concentrations, and a single stageprocess, for different temperatures for both the double stage processand the single stage process. As will be noted, the single stage processis sensitive to temperature and is only significantly reactive attemperatures above 60° C. The double stage process, even at lower nitricacid concentrations, is much less sensitive to temperature.

The process of the invention, as illustrated, can effectively remove asubstantial portion of the pyritic sulphur content of coal, withoutaffecting the integrity of the coal.

1-17. (canceled)
 18. A process for reducing the pyritic sulphur contestof coal, the process including reacting the coal with a ferric ionsolution to oxidise the pyritic sulphur to elemental sulphur; separatingthe coal, which includes elemental sulphur from the ferric ion solution;reacting the coal and the elemental sulphur in an acidic solution withnitric acid to oxidize the elemental sulphur, forming sulphuric acid;and separating the coal from the acidic sulphuric acid containingsolution to provide coal with a reduced pyritic sulphur content.
 19. Theprocess as claimed in claim 18, in which the coal is reacted with theferric ion solution at a temperature of less than 100° C.
 20. Theprocess as claimed in claim 18, in which the ferric ion solution isselected from the group consisting of a ferric sulphate solution, aferric chloride solution, a ferric nitrate solution and mixturesthereof.
 21. The process as claimed in claim 18, which includesadjusting the pH of the ferric ion solution to be acidic.
 22. Theprocess as claimed in claim 21, in which the pH of the ferric ionsolution is maintained at less than
 1. 23. The process as claimed inclaim 18, in which the nitric acid used to oxidise the elemental sulphuris at a concentration of less than 10% by mass.
 24. The process asclaimed in claim 18, in which the elemental sulphur is reacted withnitric acid at a temperature of less than 71° C., but at least 40° C.25. The process as claimed in claim 19, in which the coal is reactedwith the ferric iron solution at a temperature of less than 90° C. 26.The process as claimed in claim 25, in which said temperature is between60° C. and 180° C.
 27. The process as claimed in claim 24, in which thenitric acid used to oxidize the elemental sulphur is at a concentrationof less than 2.75% by mass.
 28. A process for reducing the pyriticsulphur content of coal the process reacting the coal with a ferric ionsolution at a temperature of less than 100° C. to oxidise the pyriticsulphur to elemental sulphur; reacting the coal and the elementalsulphur in an acidic solution with nitric acid at a concentration ofless than 10% by mass to oxidise the elemental sulphur forming sulphuricacid; and separating the coal from the acidic sulphuric acid containingsolution to provide coal with a reduced pyritic sulphur content.
 29. Theprocess as claimed in claim 28, in which the coal is reacted with theferric ion solution at a temperature of less than 90° C.
 30. The processas claimed in claim 29, in which said temperature is between 60° C. and80° C.
 31. The process as claimed in claim 28, in which the ferric ionsolution is selected from the group consisting of a ferric sulphatesolution, a ferric chloride solution, a ferric nitrate solution andmixtures thereof.
 32. The process as claimed in claim 28, which includesadjusting the pH of the ferric ion solution to be acidic.
 33. Theprocess as claimed in claim 32, in which the pH of the ferric ionsolution is maintained at less than
 1. 34. The process as claimed inclaim 28, in which the elemental sulphur is reacted with nitric acid ata temperature of less than 71° C., but at least 40° C.
 35. The processas claimed in claim 28, which includes, after reacting the coal with aferric ion solution, separating the coal, which includes elementalsulphur from the ferric ion solution before reacting the coal and theelemental sulphur with nitric acid.
 36. The process as claimed in claim28, in which the nitric acid used to oxidize the elemental sulphur is ata concentration of less than 2.75% by mass.